Create forward compatibility layer for new configuration format

[alcaeus]

3.0 brings a new configuration format to allow for multiple namespaces. To avoid a hard BC break, we should add a forward compatibility layer to 2.3 that allows people to change the configuration to the new format. However, since 2.x only supports a single namespace, the config reader should throw an exception when more than one migration namespace is configured.

This also entails deprecating the old configuration format for removal.